Platform-type wheelchair lifts of the type disclosed in U.S. Pat. No. 4,058,228 to Hall have been used extensively in passenger vehicles, particularly urban buses. Platform lifts of the type disclosed in the Hall patent are typically installed in one of the existing stairwells in a vehicle. These lifts generally comprise a platform which is slidably mounted in the vehicle beneath the stairwell so as to define the bottom step of the stairwell when in the retracted position and so as to provide a platform which projects outwardly from the stairwell when in the extended position. The platform is attached via a parallelogram linkage assembly to a carriage. The latter is slidably mounted in a pair of opposing channel members and is caused to move between retracted and extended positions by a chain drive assembly which is actuated by a hydraulic or pneumatic linear actuator. The parallelogram linkage assembly is designed to cause the platform to move between upper and lower positions relative to the carriage when the platform is in the extended position. The parallelogram linkage includes two pairs of parallel linkage arms, each arm having a proximal end which is pivotally mounted to the carriage and a distal end which is pivotally mounted to the platform.
The linkage arms are caused to pivot about their proximal ends, thereby causing the platform to move between upper and lower positions, by a sprocket and chain drive assembly. This assembly includes a toothed driving sprocket coupled with the proximal end of one of the parallel arms so as to rotate with the arm and a driven toothed sprocket attached to the proximal end of the other arm so as to rotate with the arm. Rotational drive is transmitted from the driving sprocket to the driven sprocket by a chain drive assembly comprising a single length of roller chain. One end of the roller chain is wrapped partially around and is attached to the driving sprocket, and the other end of the roller chain is wrapped partially around and is attached to the driven sprocket. The driving sprocket is driven by a separate chain and sprocket drive assembly which is actuated by a linear actuator. In an alternative embodiment of the above-described chain drive assembly, two lengths of roller chain are employed, each of which is wrapped around a respective one of the sprockets. Two turnbuckles are provided for coupling the ends of the roller chains together so as to form a continuous flexible drive member. A preload can easily be applied to a chain assembly of the alternative embodiment by appropriate adjustment of the turnbuckles.
Platform lifts of the type described above have been used extensively in urban buses with very favorable results. Recently, the need has arisen (a) to increase the length of the parallel linkage arms of known platform lifts, (b) to increase the size and hence weight of the platform of known platform lifts, and (c) to accommodate heavier loads on known platform lifts. However, due to space limitations inherent in the design of the above-described platform lifts, it has not been possible to accommodate roller chain in the chain and sprocket drive assembly of such lifts of a size and hence strength sufficient to permit the lift to be modified in the manner described in the preceding sentence.
Under conventional operation, the parallel arms of platform lifts of the type disclosed in U.S. Pat. No. 4,058,228 are caused to travel between a downwardly projecting position, through a horizontal position, to an upwardly projecting position. As the arms travel through the horizontal position, the tendency exists for the platform of the lift to "buck" or bounce. Such "bucking" is believed to occur due to stretching of the inherently flexible roller chains used in the chain and sprocket drive assembly which arises when the load applied to the roller chains is shifted from the linkage assembly to the chains as the parallel arms move from the upwardly projecting to the downwardly projecting positions, or vice versa. As those who have experienced anomalous vibration or bouncing of elevators can appreciate, such "bucking" of the platform lift can be very disconcerting to a wheelchair occupant positioned on the lift.
In an attempt to minimize the "bucking" of the platform of conventional platform lifts which occurs as the parallel arms travel through the horizontal position, a significant preload has been applied to the roller chains. Although such preloading does reduce the tendency of the platform to "buck," it simultaneously increases the load on the bearings and the strain on the chain, sprockets, and other components of the chain and sprocket drive assembly. Also, the friction between movable components of the chain and sprocket drive assembly is increased as a consequence of such preloading. Such increases in load, strain, and friction result in added maintenance costs and reduced product longevity.
In fields of technology completely unrelated to platform lifts, leaf chains have been used for transmitting motion from one movable element to another. For instance, U.S. Pat. No. 4,197,766 discloses a counterbalanced pumping system comprising a vertically movable pump member, a counterweight, and a leaf chain supported on a plurality of pulleys for transmitting motion between the pumping member and the counterweight. U.S. Pat. No. 4,526,251 discloses a leaf chain designed for use in a lift truck for transmitting motion from one end of a hydraulic cylinder to the fork of the lift truck. In the chain drive assemblies of both U.S. Pat. Nos. 4,197,766 and 4,526,251, the ends of the leaf chains are secured to fixed or movable members, as the case may be, and the length of the leaf chains are supported by and passes back and forth over one or more sheaves or pulleys.
Although in general leaf chain is not designed to transmit positive drive, it is known to use leaf chain in this manner. U.S. Pat. No. 4,058,021 comprises a drive assembly comprising a leaf chain having link edges which engage specially formed flutes on an associated pulley. As indicated in U.S. Pat. No. 4,058,021, only a limited quantity of torque may be transmitted between the chain and pulley before the chain will slip with respect to the pulley.
It is also known to use chain in a spring-biased counterweight assembly for reducing the force required to raise a railroad passenger car stairway assembly. Such use of a chain is disclosed in U.S. Pat. No. 2,154,107, although the type of chain employed, i.e., roller versus leaf chain, is not disclosed.
Although the use of leaf chains in a wide range of mechanical contexts is well known, as evidenced by the patents discussed above, platform lifts of the type disclosed in U.S. Pat. No. 4,058,228 have, since their inception, suffered from the lack of chain strength and "bucking" problems discussed above. Such drawbacks of these platform lifts have gone uncorrected for over ten years, in spite of extremely widespread use of such platform lifts.
Thus, a strong need exists for a sprocket and chain drive assembly for causing the parallel arms of platform lifts of the type disclosed in U.S. Pat. No. 4,058,228 to Hall to move between the upper and lower positions which (a) is stronger than the sprocket and chain drive assemblies currently employed, (b) does not cause the platform to "buck" as the parallel arms move through the horizontal position, and (c) does not require excessive preloading of the roller chain links of the sprocket and chain drive assembly to avoid such "bucking" of the platform.
As discussed previously, leaf chain typically is used as a high strength tensile member. In order that every leaf in each set of leaves (the parallel link plates in each pitch of chain) carries an equal amount of a load, the distance between pin holes in each of the leaves must be precisely the same. This equalizes stresses in the ends of the leaves and loads the pin in even shear instead of bending. However, while pin hole spacing is closely controlled, precise centering of the pin holes in the circular ends of the leaves is not done. Thus, eccentric leaf ends occasionally result. Moreover, irregularities may occur during the forming of the leaves. Leaves typically are stamped from sheet metal. This process is prone to size variations and to leaf edges that are "broken."
When eccentric and off-size leaves are assembled into a set, one or more leaf edges may protrude substantially laterally from the rest of the set. As long as the leaf chain is subjected only to a straight tensile pull, the lack of precision of the link edges is of no consequence. However, in applications in which the leaf chain changes direction around a sheave, or is anchored to a sheave and partially wrapped around it, any leaf of the chain that protrudes inwardly against the sheave will carry a disproportionate amount of the load. This raises the tensile stresses in the leaf and induces bending into the associated pin. A sheave that will enable all leaves of the chain to carry a load equally, despite irregularities in the leaves, is desirable.